This invention relates to a skid control system and more particularly to an improved fail safe bypass for such a system.
Most skid control systems embody a modulator that is operable upon receipt of a predetermined signal for reducing the effect of the actuating source on a wheel brake to prevent skidding under predetermined conditions. The modulators normally include an element that is moved between a normal braking position and a skid control position in response to a predetermined signal. One common actuator employed for operating such elements between its positions is a vacuum motor. Such motors normally hold the element in its normal braking position by the exertion of vacuum on one side of the power element of the vacuum actuator. This means that, in the event of failure of the vacuum circuit, the skid control modulator may move to a skid control position at an unnecessary or undesired time. This can result in the loss of braking ability.
Various types of devices have been proposed for use in combination with such vacuum actuated skid control systems for fail safe operation. Because the vacuum motor is used to move the element of the modulator between its positions during skid control operation, the fail safe device should not sense such operation as a failure and disable the modulator. Normally failures have been sensed by measuring the time interval when the power element of the vacuum actuator is in its skid control position. An excessive time duration in this position is sensed as failure and the fail safe mode is initiated. Such devices have many disadvantages. Particularly, the devices are slow to operate in that they do not immediately sense failure of the system.
It is, therefore, a principal object of the invention to provide an improved fail safe device for a vacuum actuated skid control modulator.
It is another object of the invention to provide a fail safe device that is directly responsive to failure of the skid control modulator.
It is a further object of the invention to provide an improved failure sensitive bypass system for a vacuum actuated skid control system.
As has been previously noted, it is important to provide a fail safe device in a skid control brake system for insuring normal brake operation in the event of failure of the skid control modulator and particularly failure of the actuating element for the modulator. As has also been noted, the actuator element, during normal skid control operation, moves between its normal braking position and its skid control position. Most of the previously proposed fail safe devices sense such movement as a failure in the system. It has, therefore, been necessary to provide a timing circuit or a disabling circuit for preventing operation of the fail safe device during normal skid control operation of the modulator. Such arrangements not only complicate the system but also give rise to the addition of other sources of failures.
It is, therefore, a further object of the invention to provide an improved fail safe device for a skid control system which senses normal skid control operation and does not disable the modulator during such normal operation.
In the skid control brake systems as previously described the modulator includes a skid control valve that is movable between an open, normal braking position and a closed, skid control position. In this latter position, the communication between the actuator controlled source and the wheel brake is prevented. It is desirable to provide a bypass circuit for permitting fluid communication between the source and the wheel brake in the event of failure of the skid control valve to be maintained in its normal braking position. The bypass valves previously proposed have been separate elements that add to the complexity and cost of the modulator assembly.
It is, therefore, a further object of the invention to provide an improved and simplified fail safe bypass valve for a skid control system.
It is yet another object of the invention to provide a fluid operated skid control system in which the skid control valve and bypass valve employ common elements to permit simplification and cost reduction.